1. Field of Invention
The present invention relates to a packet transmitting method. More particularly, the present invention relates to a packet transmitting method of wireless network.
2. Description of Related Art
In recent years, the wireless network systems, such as the WLAN, WMAN, have been developed, and the network is sequentially established to provide service. Among others, the packet voice and video are regarded as one of the most important application services on the wireless network system. However, how to provide the real-time communication services on the wireless network remains a challenge. For example, most of the wireless handheld devices are powered by batteries, so the power consumption issue needs to be overcome. Furthermore, most of the video and audio data of the real-time communication system are generated in specific time periods, and most of them are small packets, which significantly affects the usage of bandwidth of the wireless network system.
Yet the wireless network system is mostly designed to transmit the data packet, rather than optimize the transmission of the real-time packet. The features of the real-time packet are not fully used in the design of the wireless network system. For example, most real-time communication allows some packet loss, so using additional bandwidth to achieve the reliable transmission is not the priority when transmitting the voice packet. Therefore, if the used capacity of the network bandwidth can be expanded and the power consumption of the handheld device can be improved, it will benefit the development of the real-time communication service over the wireless network environment.
The operation flow of Standard 802.11 is shown in FIG. 1. When the Mobile Station (MS) transmits the packets, if the wireless medium is being used by other MS, the MS will wait until the wireless medium is not used, and wait for a DCF Interframe Space (DIFS), then begin to count down the contention window (CW). When the countdown reaches 0, the MS begins to transmit the control packets, such as the RTS, CTS, or the data or fragmentation packets (such as Frg#1 and Frg#2). When a correct packet is received at the receiving end, an ACK packet will be sent to the transmitting end after a Short Interframe Space (SIFS).
Referring to FIG. 2, it is a systematic architectural view of a wireless network using real-time communication service. As shown in FIG. 2, at one end of the wireless network, one or more wireless network devices 202, 204 are connected with the cable network end through the wireless network access point 210 to access the internet 230. Taking the uplink real-time packet as an example, the packet is firstly transmitted to a router 220 in the same local area network through the wireless network access point 210, and then the router 220 transmits the packet to the correspondent node 240 at the other end over the internet network, wherein the correspondent node 240 communicates in real-time with the internet network. While taking the downlink real-time packet as an example, the packet is firstly transmitted to the router 220 over the internet 230, and then the router 220 transmits the packet to the wireless network device 202 or 204 over the wireless network access point 210.
Based upon the conventional arts, when the wireless network environment is used to transmit the real-time packet, the simplest way is to keep the network card on permanently, so that the packet can always be transmitted and received. Whereas, the network card of such a design still consumes power even if it does not transmit packets. A method for reducing the power consumption is to keep the wireless network in a sleeping mode, and then activate the wireless network devices to transmit packets when the packet needs to be transmitted. Such method is usually called as PS-Poll transmission mechanism. Referring to FIG. 3, it is a schematic view illustrating the packet timing diagram and the corresponding power consumption when the real-time packet is transmitted over the wireless network by the PS-Poll mechanism. As shown in FIG. 3, the wireless network devices can transmit the uplink and downlink real-time packets after being linked and registered. After the uplink real-time packet is sent, the wireless network devices is still required to wait for the ACK packet of the wireless network access point (AP) since it should determine whether the packet needs to be re-transmitted or not. Moreover, when the wireless network device is in a power-saving mode, for the downlink real-time packet, the PS-Poll is used to fetch the packet stored in the buffer region of the wireless network access point. Similarly, in order to facilitate the wireless network access point to determine whether the packet should be re-transmitted or not, the wireless network devices is further required to send the ACK packet to the wireless network access point, so as to confirm whether the packet is received correctly. Thus, for the purpose of achieving a reliable transmission, it is inevitable to use two SIFSs and two ACK packets additionally, and therefore the power consumption and used capacity of the network bandwidth cannot be fully improved.
If the Unscheduled-Automatic Power Saving Delivery (U-APSD) mechanism defined by the 802.11e is used to transmit the real-time packet, the power consumption of the PS-Poll transmission mechanism can be further improved. Referring to FIG. 4, it is a schematic view illustrating the packet timing diagram and corresponding power consumption when the real-time packet is transmitted over the wireless network by the U-APSD mechanism. Compared with FIG. 3, the mechanism shown in FIG. 4 is similar to that of FIG. 3, except that a PS-Poll packet is omitted, and is not described in detail herein.
To achieve a preferable power-saving effect, many related medium control techniques are further developed in addition to the Power-saving Delivery mechanism, including the techniques of reducing the sync signals and space between the MS and the wireless network access point; alleviating the problems of hidden terminal, reducing the interference and collision and the probability of packet re-transmission; observing the current transmission status of the wireless medium and reducing the additional transmitting and receiving actions; predicting the time point at which next packet is transmitted and received, based on data statistics or forecasts; analyzing the long-term or short-term transmission for the packet to control the transmitting and receiving, so as to avoid the additional power consumption; modifying the transmission mechanism of the standard CSMA/CA; reducing the control overhead in transmitting the fragmentation packets; adding the protective mechanism and reducing the probability of collision in the transmission procedure of the long packet; adding or modifying the feedback message to help adjust the transmitting power; or reducing the re-transmission of the packet data units or the ACK packet. Furthermore, there are some other power-saving techniques, such as the technique of reducing the measurement in the sleep mode; the technique of reducing the power consumption in receiving the short control packet, the power-saving mechanism related to the QOS, the technique using Time Division Multiple Access (TDMA), the technique using Packet Reservation Multiple Access (PRMA), and the technique of reducing the transmission of the control message of the networks.
However, many of the above power-saving mechanisms modify the transmission mechanism of 802.11, and are not compatible with the standard. Some of the above mechanisms cause different power-saving effects due to the limitation of the measurement or evaluation results. Some cause power consumption of another part for the purpose of saving the power consumption of a certain part. And some cause other effects, such as a low transmission throughput or network off-line, in order to achieve the power-saving effect.
In other words, the present techniques for saving power still have drawbacks. Especially, some of the power-saving mechanisms may cause inconvenience in usage due to the incompatibility with the standard. Therefore, the conventional art cannot overcome the problems encountered in the real-time communication effectively.